Spelling depends on various things. If it's followed by another number, then its singular: quatre-vingt-trois. Otherwise, it's plural, quatre-vingts. Unless it's an ordinal / noun, J'habite au quatre-vingt rue de la Paix.

Pronunciation is the same, unless there is a liaison: elle a quatre-vingts ans = there will be a 'z' sound between vingts and ans.

FYI, I'm a native speaker and I had to look the spelling part up to be sure. There are other rules with other exceptions for cent, mille, million, milliard.